This invention relates generally to compositions used for acoustic damping, such as but not limited to those used in damping wedges for ultrasonic probes.
Ultrasonic probes having phased array transducers inject acoustic waves into an object under test at an oblique angle to inspect the test object for flaws of defects. When the oblique angle is larger than the first critical angle, according to Snell's Law, the longitudinal waves will disappear, and only the newly converted shear waves will propagate in the object under test. The simplicity of one pure wave mode benefits the ultrasonic inspection greatly. The acoustic waves used in the object under test are preferably shear waves. A wedge with an angle larger than the first critical angle is usually attached to the transducer to generate shear waves in objects under test. However, a portion of the longitudinal waves generated by the transducers are reflected from the wedge body-test object interface. If the multiple reflections of these longitudinal waves in the wedge are not eliminated before being received by the array transducers, the longitudinal wave echoes produce noise in the image generated from the received ultrasonic shear wave echoes.
Shear wave ultrasonic probes typically have a wedge body connected to the ultrasonic transducers on an angled surface relative to the wedge body surface that will contact an object under test, and a damping wedge fit over the front side of the wedge body opposite the transducers. The damping wedge is provided for the purpose of reducing the longitudinal wave reflections or echoes as much as possible from the received ultrasonic signal. The damping wedge must both attenuate ultrasonic signals, measured in dB attenuation per inch (dB/in.), and match the impedance of the wedge body, measured in MRayl, while being sufficiently rigid to be machined and to maintain its shape, which can exclude many substances from consideration for use making damping wedges.
The attenuation and/or impedance matching of the damping wedge are often not optimal. FIGS. 1 and 2 illustrate the problems associated with the prior art. FIG. 1 shows the situation when the attenuation by the damping wedge 10 is not sufficient. In this case, the ultrasonic probe 50 has transducers 30 which produce longitudinal (L) waves. A portion of the longitudinal waves (L) are converted to shear waves (S) when they are incident into the interface between the wedge body 20 and the object under test 40. The converted shear waves (S) pass through the object under test 40. Meanwhile, longitudinal waves (L) reflect from the wedge body-test object interface 35, into the prior art damping wedge 10 and then the echoes return through the wedge body-damping wedge interface 15 and wedge body 20 are still strong enough to be received by the transducers 30. When impedance matching between the prior art damping wedge 10 and the wedge body 20 is poor, the longitudinal wave (L) reflects and echoes strongly from the wedge body-damping wedge interface 15 with the prior art damping wedge 10, as illustrated by FIG. 2.
Each of these problems results in unwanted longitudinal wave echoes being received by the transducer and generating a noise signal. As the ultrasonic frequency becomes lower, the noise increases due to the fact that the attenuation from the damping has a direct relationship with frequency. Operating frequencies below 4 MHz are desirable to use for inspecting certain objects with ultrasonic, but noise from non-attenuated longitudinal waves interferes with the inspection when using known damping wedge material compositions. For example, lower frequencies, around 1.5 MHz to 2 MHz, provide greater penetration depth than frequencies above 2 MHz, which is required in certain applications. But, known damping wedge material compositions are insufficient to attenuate the noise signal from longitudinal waves.
Ultrasonic probe designers can use three mechanisms to reduce noise from longitudinal waves—scattering, absorption, and ultrasonic geometric divergence. Scattering is done by adding fillers to damping wedge materials compositions for reflecting the longitudinal waves non-coherently. Absorption reduces wave strength by converting wave energy to heat, and is related to the viscoelasticity of the material used. Geometric divergence utilizes v-grooves formed in the wedge body-damping wedge interface.
Available damping wedges do not provide sufficient damping of longitudinal waves at lower frequencies, such as around 2 MHz. FIGS. 3 and 4 illustrate the frequency spectrum response and attenuation provided by a prior art damping wedge composed of Epoxy 303 available from Mereco Technologies Group, Inc. for a 24 inch and 48 inch blocks, referred to herein as “thin” and “thick” blocks, immersed in water. The thick block, for example, has a frequency response range of about −40 dB between 4 MHz to 2 MHz operating frequency. Both the thick and thin blocks exhibit a total attenuation of about −80 dB/in. at 4 MHz, only −40 dB/in. at 2 MHz and slightly more than −20 dB/in. at 1 MHz. As seen in FIG. 4, the attenuation function is substantially linear with a slope of about −17.0 (dB/in.)/MHz.
Application size constraints also limit the dimensions of the damping wedge, and thereby the amount of damping material available to attenuate unwanted acoustic waves. For example, some applications limit the size of the damping wedge to less than one half inch thick. Thus, simply adding material to a damping wedge is not an option for making the damping more effective. Damping wedge material compositions must also be sufficiently rigid that they can be machined using saws, grinders and other tools.
Accordingly, a need exists for an improved ultrasonic damping wedge material composition for making damping wedges which are effective at relatively low ultrasonic frequencies. Further, a method for designing effective ultrasonic damping wedges is needed to optimize impedance matching and attenuation of unwanted acoustic waves. The damping wedge material composition must be rigid enough that it can be machined to useful shapes.